Time division multiplexing (TDM) techniques are commonly used in telecommunication systems to increase the amount of information that can be carried on a transmission line. For example, TDM techniques are used in the internal architecture of private branch exchanges (PBXs) and in the transmission of digital signals over telecommunication lines to maximize the amount of data which can be handled by these systems.
The majority of contemporary telecommunication systems use a TDM arrangement in which each off-hook connection (i.e., when the telephone line is in use) is allocated a specific periodic time interval for information transfer. The periodic time interval is generally equal to eight times the data bit rate of the connected device, allowing a word (8 bits) of information to flow during each periodic time interval assigned to that device.
Conventional time division multiplexing (TDM) arrangements are designed to operate with standard carrier TDM arrangements which have the capability of handling multiple channels on the same transmission line, such as T1 (24 channels), E1 (32 channels), 64-slot (64 channels), and 128-slot (128 channels) arrangements. Each TDM arrangement consists of a fixed length frame used to transmit data. The frame is divided into a predetermined number of time slots, each representing a different channel. For example, a T1 line is designed to carry 24 voice-grade channels with data from each channel broken down into 8 bit words. Combining 24 voice channels (24 channels times 8 bits per channel equals 192 bits) into a serial bit stream and including a framing bit yields a frame size of 193 bits. E1, 64-slot, and 128-slot TDM arrangements operate according to similar principles, with the exception that a framing bit is not used.
Internet service providers (ISPs) offer Internet access to home users by allowing home users to call local telephone numbers and use modems connected to the user's computer to communicate with modems located at the ISP. The ISP then processes the information received at its modems to generate data streams which can be placed on standard telecommunication lines, such as a T1 line, and transfers the data received by the modems located at the ISP to a telephone company central office for connection to the Internet. As the popularity of the Internet expands, ISPs will require telecommunication devices which allow data received from users through a large number of modem connections to be processed and placed on Internet connection lines that make the most efficient use of the transmission lines in a minimal amount of hardware space.
FIG. 6 is a block diagram illustrative of a prior art interface circuit 60 between multiple modem signals, represented by channel 1 through channel n, and a time division multiplexing (TDM) bus 68. In this simple multi-channel interface, tri-state buffers 62 are used to control access to the TDM bus 68. Controls 66 are used to control when the tri-state buffers 62 are active. When the TDM bus 68 is ready to accept information, the TDM bus 68 will signal the controls 66 to allow data to flow. The controls 66 will activate the tri-state buffers 62 to allow data to flow onto the TDM bus 68. In order to eliminate the potential for conflicts, the controls 66 are individually programmed to send data during a specified time slot assigned to the channel corresponding to the individual control. At all other times, the controls 66 will tri-state the tri-state buffers 62. When the tri-state buffers 62 are tri-stated, no data is allowed to flow from the channels to the TDM bus 68, leaving the TDM bus 68 idle for other devices to access.
The setup shown in FIG. 6 requires separate controls 66 for each tri-state buffer 62. The number of components required for this arrangement requires area which makes it difficult to develop more compact designs. Also, this arrangement uses separate controls 66 to activate the individual tri-state buffers 62 and control the duration of the activation, requiring processing power which could be used for other functions.